The use of CoPt as a magnetic material for fabricating both bulk and deposited magnets is well known in the art. For example, because of its magnetic properties, thin film CoPt has been utilized as an anhysteretic recording medium in magnetic disks and as a permanent magnet in the biasing of a magnetoresistive head. For both longitudinal recording and permanent magnets, the preferred CoPt film is a polycrystalline structure consisting of close packed hexagonal crystallites, whose C-axes lie in or have an appreciable component lying in the plane of the film.
The external magnetic field produced by a permanent magnet is proportional to the product of the energy product (HB) and the volume of the magnet. The energy product (HB) of a magnetic film is directly affected by the magnetic induction value B, coercivity (Hc), squareness ratio (SR) and saturation magnetization (Ms). A recording head (write head) must be capable of creating a magnetic field strong enough to magnetically transfer information to a tape. Typically, the ability of a particular recording head to transfer information to a particular tape depends upon the energy product of the head relative to the tape. As mentioned above, the energy product of a film is largely dependent upon its coercivity. Generally, to record from a write head to a tape, the coercivity of the film on the write head must be roughly twice that of the tape. Thus, to record from a write head to a magnetic tape having a coercivity of about 1000 Oersteds (Oe), the write head must have a film deposited thereon having a coercivity of about 2000 Oe, assuming the relative thicknesses of the two films are roughly the same. Consequently, if the coercivity of the write head magnetic film is less than 2000 Oe, it would probably have to be deposited at a greater thickness to be effective.
The methods of the prior art teach a wide variety of deposition techniques for CoPt magnetic films. For example, CoPt deposition can be achieved with or without the use of an underlayer, which may be chosen from various different materials, such as Cr or W. Also, the composition of the CoPt sputtering targets and the temperature of the substrate may be varied. Although the above parameters and others have been varied to yield CoPt films having various properties, all of the prior art films have exhibited relatively larger decreases in coercivity with increasing film thickness, especially at thicknesses greater than 300 nm. Furthermore, the decreasing coercivity effect is accentuated when the prior art films are deposited on rotating substrates. On rotating substrates, using the methods of the prior art, it is difficult to achieve a coercivity of 1800 Oe for magnetic films having a CoPt thickness greater than 400 nm.
For example, U.S. Pat. No. 4,902,583 to Brucker et al., the disclosure of which is hereby incorporated by reference, discloses the deposition of cobalt platinum magnetic films having a CoPt thickness of 300-1,000 nanometers. The films are deposited onto non-moving substrates and result in coercivites as high as about 2,000 Oe. This is achieved by optimizing the "throw distance" to ensure adequate "thermalization" of the depositing CoPt atoms, i.e. the reduction of particle energy to about 0.039 electron volts. Brucker states that the coercivity of these magnetic films decreases as film thickness increases.
The decrease in coercivity inherent with increasing thickness in prior art CoPt films has prevented sputtered CoPt films from being used in some applications, for example, as a permanent magnet for biasing a magnetoresistive head. Prior attempts to increase the external magnetic field of a CoPt film by increasing the film thickness have been defeated by the rapid decrease in coercivity inherent with increasing film thickness. U.S. Pat. No. 4,596,646 suggests a solution to this problem by stacking deposited CoPt films of thickness 100-120 nm, separated by silicon dioxide insulating layers, to attain a magnet of requisite volume. However, as the number of SiO.sub.2 /CoPt layers increases, so does the relative cost of the deposition process.
U.S. Pat. No. 4,778,582 to Howard teaches the sputter deposition of Co77 Pt Cr film over a Cr80 V underlayer, which is deposited in an atmosphere of argon and hydrogen. Unlike Applicants' invention, Howard's films are not CoPt and they do not exhibit increased coercivity with increased hydrogen content, as evidenced by FIG. 1 therein. Further, unlike Applicants' invention, Howard teaches that his films require a hydrogenated carbon overcoat. The reference discloses films having a coercivity approximating 1600 Oe, and states that the hydrogen present in the sputtering atmosphere has no adverse affect on the magnetic properties of the recording disk.
Thus, there continues to be a need for a CoPt magnetic film which, when deposited at thicknesses of 300 nm and higher, still has a high coercivity, i.e., above about 2100 oersteds, and preferably above about 2300 oersteds. Further, there continues to be a need for a method to deposit high coercivity CoPt films, especially those having a coercivity above about 2,000 Oe, on rotating substrates, such as, for example, cylindrical recording drums.